The present invention relates to a process for continuously preparing methyl mercaptan from hydrogen sulphide and methanol in direct connection with the preparation of hydrogen sulphide.
Methyl mercaptan in particular is an industrially important intermediate, for example for the synthesis of methionine and for the synthesis of dimethyl sulphoxide and dimethyl sulphone. It is currently prepared predominantly from methanol and hydrogen sulphide by reaction over a catalyst composed of aluminium oxide. Methyl mercaptan is commonly synthesized in the gas phase at temperatures between 300 and 500° C. and at pressures between 1 and 50 bar.
In addition to the methyl mercaptan and water formed, the product gas mixture comprises the unconverted methanol and hydrogen sulphide starting materials and dimethyl sulphide and dimethyl ether as by-products, and also small amounts of polysulphides (dimethyl disulphide). Gases inert in the reaction, such as carbon monoxide, carbon dioxide, nitrogen and hydrogen, are also present in the product gas. The methyl mercaptan formed is removed from this reaction mixture. The reactant gas mixture comprises predominantly hydrogen sulphide and methanol in a molar ratio between 1:1 and 10:1.
As explained in DE-1768826, the methyl mercaptan formed is removed from the product gas mixture in several distillation and wash columns at temperatures between 10 and 140° C. The further product streams obtained are excess hydrogen sulphide, methanol, inert gases such as carbon monoxide, carbon dioxide, nitrogen and water. The wash liquid used is preferably methanol. Excess hydrogen sulphide is recycled into the reactor as so-called cycle gas. In addition to hydrogen sulphide, the cycle gas also comprises methanol, methyl mercaptan, dimethyl sulphide and organic components, and consumed hydrogen sulphide and methanol are replaced by supplying fresh media.
The overall process for methyl mercaptan preparation can be divided into two sections. The first section comprises the workup of the reactant gas mixture and its conversion to methyl mercaptan. The second sector includes the separation of the product gas mixture to obtain methyl mercaptan and recycling of the unconsumed feedstocks, and also the disposal of wastewater and offgases.
For economic viability of the process, minimum capital and operating costs are required. Here, the cost for apparatus and machines in particular, but also the energy demand for the synthesis and workup of the reactant gas mixture, constitutes a high cost factor. For example, large electrical outputs are required for the operation of compressors and of heating and cooling circuits.
According to FR 2477538, methyl mercaptan is prepared by compressing fresh hydrogen sulphide gas to 11 bar in a compressor. Thereafter, cycle gas which comprises hydrogen sulphide, dimethyl sulphide, methanol and small amounts of methyl mercaptan and has been recycled from the process is added to the compressed hydrogen sulphide to form the reactant gas mixture. A preheating oven raises the temperature of the gas mixture after the compression to 510° C.
In DE 19654515 too, the compression of the reactant gases to operating pressure is described preferentially in two stages, for example with a two-stage compressor, the gas mixture being compressed in the first stage to an intermediate pressure and in the second stage to the operating pressure. The methanol can be injected directly into the first compressor stage. The reactant gas mixture thus obtained is then heated first to an initial temperature of 150 to 250° C. and then further to the reaction temperature. At this temperature, the reactant gas mixture passes into the reactor for the formation of methyl mercaptan. Owing to the temperature limit in a compression, the temperature after the second compressor stage can be raised to a maximum of 140° C.
This means that the entrance temperature of the hydrogen sulphide before the compression must, for example, be at ambient temperature. Consequently, the hydrogen sulphide prepared beforehand at high temperature must first be cooled and, after the compression, heated again to obtain the reaction temperature for the formation of methyl mercaptan. This cooling and repeated heating requires numerous heat exchangers and high energy costs. Moreover, the hydrogen sulphide for compression should not comprise any impurities or even solids, in order not to damage the compressor.
The synthesis of hydrogen sulphide from the elements hydrogen and sulphur is effected typically by introducing a hydrogen into liquid sulphur and a subsequent reaction chamber in the gas phase. Both catalysed and uncatalysed processes are known.
The industrial production of hydrogen sulphide from the elements proceeds according to Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, 2002, at temperatures of 450° C. and a pressure of 7 bar.
CSSR 190792 describes a process variant for preparing hydrogen sulphide, in which high reaction temperatures are avoided by a comparatively complicated series connection of a plurality of reactors. High temperatures are avoided there especially because of corrosion problems.
GB 1193040 describes the uncatalysed synthesis of hydrogen sulphide at relatively high temperatures of 400 to 600° C. and pressures of 4 to 15 bar. It is stated that the required temperature is determined by the pressure at which the synthesis should proceed. At a pressure of 9 bar, 500° C. are accordingly required.
Overall, there are numerous publications with different catalysts for preparing hydrogen sulphide. For instance, U.S. Pat. No. 2,214,859 describes the use of several different metal oxides and metal sulphides with high conversions of hydrogen. U.S. Pat. No. 2,863,725 describes the use of catalysts such as molybdenum sulphide, cobalt oxide or cobalt molybdate bound to supports such as bauxite or aluminium oxide, in order to prepare substantially sulphur-free hydrogen sulphide.
An important point in the preparation of hydrogen sulphide from sulphur and hydrogen is in particular the temperature control. High temperatures are necessary in order to achieve an equilibrium state in which a molar hydrogen:sulphur ratio in the gas phase of about 1:1 is established. Only this enables the synthesis of pure hydrogen sulphide. With increasing pressure, the temperature has to be increased greatly in accordance with the vapour pressure curve of sulphur, in order to achieve the desired molar ratio of 1:1 in the gas phase. In this context, even small differences in the pressure of, for example, 1 bar or less are of great significance.
It is an object of the invention to provide a novel process for preparing methyl mercaptan.